This invention relates generally to methods and apparatus utilizing agglomeration to improve the performance of baghouses installed in series with an electrostatic precipitator, and to systems utilizing such methods and apparatus.
In some known industrial plant air pollution control systems, an electrostatic precipitator and fabric filter are combined to allow a baghouse to operate at a higher air to cloth ratio than does a fabric filter that experiences a full dust burden of a process gas stream. The electrostatic precipitator is intended to reduce the dust burden reaching the fabric filter. As a result of the reduced dust burden, some designers increase the air to cloth ratio of the fabric filter, enabling the fabric filter to be relatively compact (i.e., less cloth area for a given gas volume). The expectation is that the baghouse can operate at an acceptable pressure drop even though significantly greater volumes of gas are forced through every square foot of cloth filter.
In practice, however, baghouses operating in series with an electrostatic precipitator to reduce particulate emissions experience high pressure drop and short bag life in comparison to conventional fabric filters. These conditions result because the electrostatic precipitator removes 95% or more of the incoming dust and essentially all coarse particles, so the dust that enters the fabric filter is extremely fine. This extremely fine dust creates a dense dust cake, which over a period of time becomes embedded in the fibers of the filtration media, causing permanent increases in pressure drop. Operators attempt to recover the pressure drop by increasing pressure used to pulse the bags and by reducing intervals between cleaning cycles. However, this mode of operation results in reduced bag life due to fabric fatigue.
Some known systems utilize a compact hybrid particulate collector (COHPAC), which is described in U.S. Pat. No. 6,514,315, “Apparatus and Method for Collecting Flue Gas Particulate With High Permeability Filter Bags,” issued to Ramsay Chang on Feb. 4, 2003 and assigned to the Electric Power Research Institute, Inc. (EPRI), Palo Alto, Calif. and other patents. In some of these configurations, fabric filters operate at an air to cloth ratio of 8 ft/min (2.4 m/min) or higher and the filters are installed in series with an existing electrostatic precipitator. COHPAC installations can experience undesirable bag blinding and pressure drop. By using a higher permeability fabric and operating at air to cloth ratios of 6 ft/min (1.8 m/min) or less (i.e., below the range stated in the EPRI patent), bag blinding and pressure drop are reduced. However, part of the cost of this reduction is a trade-off with emission compliance.